The present invention relates to a highly directional optical system which is capable of separatively detecting a plane wave with a two-dimensional intensity distribution that enters it from a predetermined direction, and which is suitable for detecting absorption distribution in a scattering object, for example, a living body. The present invention also relates to an optical sectional image forming apparatus which is capable of imaging with high resolution information light that is wrapped obscurely in scattered light.
Since the discovery of X-rays, techniques of externally observing the inside of a living body (e.g., human body) without damaging it (i.e., a bloodless or non-destructive measuring method) have been strongly demanded and developed in the field of biology, particularly in the medical field. These techniques employ gamma rays and X rays, which have the shortest wavelengths among the electromagnetic waves, and radio waves, which have the longest wavelengths among them. The technique that employs the former has already been put to practical use as X-ray CT, and the technique that employs the latter as NMR-CT (Magnetic Resonance Imaging, i.e., MRI).
On the other hand, fewer attempts have been made to apply spectroscopy that deals with the measurement and analysis of ultraviolet, visible, near infrared and infrared spectra, which is widely employed in the fields of physics and chemistry, to in vivo measurement. This is because biometry that employs light, particularly the one that utilizes the process of absorption or emission of light has many problems left unsolved in terms of "quantitativeness", which is the most basic matter. This is the reason why reproducibility is inferior and reliability is low in regard to the absolute values obtained in measurement that is conducted at the present time by using, for example, an apparatus that measures reflected spectra with a solid-state device, or a highly sensitive TV camera.
In a case where light is applied to a scattering object such as an organic tissue, if the light is received face to face at 180.degree., it is possible to take out rectilinearly propagating light to some extent. However, the spatial resolving power is not very high in the present state of art.
The difference in the spatial resolving power between X-rays and light cannot be made up in the present state of art. However, employment of light rays, particularly near infrared rays will enable imaging of a tissue oxygen concentration from the hemoglobin in the blood. These light rays will give information which is different from that obtained by other techniques such as NMR-CT and X-ray CT.
For example, if an object O in FIG. 1 is a substance which does not contain a large amount of scattering matter and which has relatively high transparency, it is possible to conduct observation in such a way that a light component of a specific wavelength is selected through a filter 340 and applied to the object O from a ring-shaped slit 341 that is placed at a focal point of a lens L.sub.1 and an enlarged image is formed on a plane P through an objective lens L.sub.2. The use of the ring-shaped slit 341 that is placed at the focal point of the lens L.sub.1 enables application of light to the object O from various directions and hence permits observation of images I.sub.1, I.sub.2. . . of the object O as viewed from various directions at a time, as shown in FIG. 2.
As for relatively thin tissues with a thickness of 3 to 5 cm, it is possible to detect light transmitted thereby. This means that "photo-roentgenography" can be used for diagnostic purposes. The women's breasts have relatively homogeneous tissues and hence readily transmit light, and it is easy to detect the light transmitted thereby (thickness: up to about 3 cm) owing to the configuration. For this reason, "photo-roentgenography" has been employed for a medical examination for breast cancer for a long time under the name of "diaphanography (lightscanning)". A conventional diagnostic apparatus for such diaphanography will be explained below with reference to FIG. 3.
FIG. 3 is a block diagram of a conventional apparatus for obtaining a light absorption distribution image. In the figure, reference numeral 401 denotes a scan head, 403 a human body, 405 a video camera, 407 an A/D converter, 409 a near infrared frame memory, 411 a red frame memory, 413 a processor, 415 a color conversion processing unit, 417 an encoder keyboard, 419 a D/A converter, 421 a printer, 423 a monitor, and 425 a video tape recorder.
Red light (strongly absorbed mainly by the hemoglobin in the blood) and near infrared light (absorbed by the blood, water, fat and so forth) are alternately applied to a part of the human body which is an object of measurement, for example, the breast, by the scan head 401 through a light guide to thereby scan the object. In the figure, light is applied upwardly from the lower side of the object. As a result, the whole breast is brightly lit up, and an image of the transmitted light is picked up by the video camera 405 and converted into a digital signal in the A/D converter 407, from which near infrared light and red light are fetched into the respective frame memories 409 and 411 through a digital switch. An intensity ratio of near infrared light to red light is computed in the processor 413 on the basis of data from the two frame memories 409 and 411. Further, color conversion and D/A conversion are executed successively, and the resulting light absorption distribution image is observed through the printer 421, the monitor 423 or the video tape recorder 425.
In this apparatus, the light rays from the scan head 410 are not parallel rays but divergent rays that diverge in the tissue (breast) as if the object were illuminated with a flashlight, and these divergent rays are received by a two-dimensional detector, i.e., a video camera; therefore, the resolving power is not very good.
One example of a system in which collimated rays are applied and received to improve the resolving power will be explained below with reference to FIG. 4.
FIG. 4 is a block diagram of a conventional apparatus that uses a collimated light applying and receiving system to obtain a light absorption distribution image.
In this example, laser light, which is used as a light source, is guided through an optical fiber 433 to illuminate an object 435 of measurement, and the transmitted light is picked up by a fiber collimator 437 and converted into an electrical signal in a detector 443, which is then sent through a pre-processing circuit 445, an A/D converter 447 and an interface 449 to a computer 451 where signal processing is executed. In this case, the optical fiber 433 for illumination and the fiber collimator 437 for detection are synchronously moved to scan the object 435 by a motor 439, thereby obtaining a light absorption distribution image of each part of the object 435 and observing it on a monitor 453.
As for the light source, a He-Ne laser of 633 nm and a semiconductor laser of 830 nm are employed for red light and near infrared light, respectively. With this diagnostic apparatus, Jobsis et al. succeeded in 1977 in detecting near infrared light transmitted by the cat's head or the human head and reported that the quantity of transmitted light varies with the respiratory condition of animals. If the size of a tissue which is to be measured is on the order of that of the cat's head, near infrared rays with wavelengths of 700 to 1500 nm enable the transmitted light to be detected satisfactorily with an illuminating light quantity of about 5 mW. This light quantity is less than 1/50 of the existing safety criterion for laser, and it is about 1/10 of near infrared rays to which we are usually exposed on the beach. The procedure is therefore considerably safe.
Incidentally, when light is applied to a living body or the like, the transmitted light is subjected to absorption and scattering by the specimen.
FIG. 5 is a graph showing Twersky's curve of scattering theory, in which the relationship between the absorbance of a red blood cell suspension and the hematocrit is determined. The graph shows the intensity of transmitted light, together with the scattering component and absorbance component of the transmitted light, obtained on illumination with laser light with a wavelength of 940 nm.
As will be understood from FIG. 5, the transmitted light involves a large scattering component superposed on the absorbance component. Since the scattering component is lacking in directivity, it includes light rays scattered from various regions, so that the resulting optical sectional image is blurred. Owing to the scattering component, the absorbance component, which is the necessary information, cannot therefore be detected with high accuracy simply by detecting the transmitted light.
FIG. 6 is a view for explanation of the optical properties of a specimen such as a living body.
In the example shown in FIG. 1, the object O contains no scattering component, that is, an object which is visual by nature is observed. In actual practice, however, a specimen 460, which is an object of observation, can be considered to be equivalent to a combination of a Rayleigh scattering object 460a which is sufficiently large in comparison to the wavelength of light, a Mie scattering object 460b which is on the wavelength order, a light transmitting information object 460c which is an object of observation and which causes the desired light absorption, a diffusing object 460d which diffuses light, a diffraction grating 460e which causes random diffraction, etc. Light that emerges from such a specimen when illuminated with a coherent plane wave through a laser optical system 461 includes Rayleigh scattered light, Mie scattered light, diffused light, random diffracted light, etc., in addition to the transmitted light, and it has heretofore been impossible to detect only the light transmitted by the information object 460c from these light rays.
FIG. 7 shows a Fresnel diffraction wave that is generated by a sinusoidal grating with a finite aperture.
When a plane wave is applied to a finite aperture, sidebands 471 and 472 are generated outside the transmitted light 470. Accordingly, it is difficult to detect the transmitted light 470 with high sensitivity for observation due to the effect of the sidebands 471 and 472.
FIG. 8 shows a luminance distribution on a plane of view that is disposed at the side of a random scattering object 480 which is remote from a light source when coherent light is applied to the object 480.
If coherent light such as laser light is applied to a scattering object such as a living body, a random diffraction image appears on the plane of view, as shown in FIG. 8(a). If the transmitted light from the scattering object 480 is focused through a lens L, as shown in FIG. 8(b), the random diffraction image makes it impossible to view an image of a region of a living body, for example, which is desired to observe with high resolving power.
FIG. 9 shows a luminance distribution of reflected rays in accordance with the condition of a plane of diffuse reflection, in which FIG. 9(a) represents it in polar coordinates, and FIG. 9(b) in rectilinear coordinates.
In the figures, reference symbol J denotes a luminance distribution of reflected rays from a plane of perfect diffusion, G a luminance distribution of reflected rays from a glossy plane, and P a luminance distribution of reflected rays from a dull plane. It will be understood from the figures that with a glossy plane a sharp peak with no expansion can be obtained in a predetermined direction, whereas with a dull plane the luminance distribution expands, and that the luminance distribution changes with the condition of the plane of reflection and hence the observation that utilizes reflected rays is greatly dependent on the condition of the plane of reflection.
As has been described above, when a sectional image is viewed by use of coherent light, the required information light is wrapped obscurely in light rays scattered by various scattering objects, so that it has heretofore been impossible to view images with high resolving power.